Some communication systems are digital systems in which binary data is transmitted over a medium using a time division multiplex (TDM) protocol. Such systems may encode the binary data as non-return-to-zero (NRZ) and may not utilize any line coding. The transmission medium may be optical fibers but may also be copper or any other type of physical media.
To ensure that low enough signal content is contained in the lower part of the frequency spectrum, either line coding or scrambling may be employed. This potentially improves the Direct Current (DC) balance of the signal and potentially reduces the risk of having long runs of Contiguous Identical Digits (CID). Some systems scramble the signal while some parts of the signal must be left unscrambled.
One example of such a system is a Passive Optical Network (PON) fiber access system. A PON is a communication system where data is transmitted bi-directionally over a fiber infrastructure, the Optical Distribution Network (ODN), in a point-to-multi-point configuration. The ODN includes an Optical Line Termination (OLT), which resides in a Central Office (CO). The OLT services a number of Optical Network Units (ONUs) or Optical Network Terminations (ONTs) residing at, or close to, the premises of the end users, typically connected in a star arrangement using optical power splitters. Since the physical medium is shared, the ONUs are scheduled by the OLT to transmit in the upstream direction in a Time Division Multiple Access (TDMA) manner.
More specifically, the 10 Gbit/s capable PON system, which is standardized by the ITU-T and referred to as XG-PON, exemplifies such a system.
In the types of systems outlined above, it is often desirable to ensure that low enough signal content is located in the lower part of the frequency spectrum. There are several reasons for this including, but not limited to:                The low frequency part of the signal may be blocked. This is commonly referred to as “AC-coupling” or “DC block”, but several other names exist as well. There are several reasons why a system may be AC-coupled, both intentional and unintentional. One example of an intentional reason is to allow for a receiver design where the received signal is compared against its average value over time. In this way, it can be detected whether the transmitted signal was a binary “one” or a binary “zero”.        Clock and Data Recovery (CDR) works best when there is high enough transition density in the signal.        
The DC balance (or DC component) of a signal is the shill of the transmitted center level. The shill is created by the ratio of the average time the signal is on compared to the time the signal is off. Any drift of the transmitted signal from the center baseline level will create a DC imbalance, which degrades the performance of the communication. This performance degradation can be quantified, for example, in terms of Bit Error Rate (BER) penalty or sensitivity penalty at the receiver.
Scrambling works by the principle of randomizing data. Calculated over time, a random data signal has good DC balance and a low probability of long runs of CID. In practice, this reduces the risk of a large amount of the signal content residing in the low frequency region of the spectrum. Scrambling works by calculating the bit-wise XOR of the input data stream with a scrambling sequence. The result of this operation, the scrambler output, is transmitted over the physical link. At the receiver, the same procedure is repeated, often referred to as descrambling. In the case of error-free transmission over the physical link, the output of the descrambling is identical to the data that was input to the scrambler.
If the data input to the scrambler is exactly synchronized to the scrambler sequence, only zeros are generated at the output. Thus, a long run of CID would be generated, which would negatively affect the DC balance of the signal. This could potentially lead to bit errors and loss of synchronization in the receiver. Additionally, it would allow a malicious user to transmit data that contains the scrambler sequence to purposely disrupt the system.
Examples of systems that utilize scrambling are GPON (ITU-T recommendation G.984 series) and the next generation 10 Gbit/s capable PON system, which is currently being standardized by the ITU-T (ITU-T Recommendation G.987 series) and herein referred to as XG-PON. In these systems, transmission is frame-based. All contents of the frame, apart from an initial part, are scrambled with a frame-synchronous scrambler (i.e., the scrambler is preset to a deterministic state at the start of each frame). Each frame contains a frame counter which increments by one for every frame. This provides timing information to devices that are connected to these systems.
One method of preventing the data input to the scrambler from being exactly synchronized to the scrambler sequence is to use a long scrambler sequence. As an example, in the XG-PON systems, the signal is scrambled with a frame-synchronous scrambler, which is generated by a 58-bit shift register operating at the line rate. If the user data contains a part of the scrambler sequence, the use of a long scrambler sequence prevents the user data from being exactly synchronized to the scrambler sequence.
Since the XG-PON frames are much shorter than the scrambler sequence, the frame counter located in the first part of the frame is used to provide an initial seed to the scrambler in each frame transmission. The seed is used to set the scrambler shift register to an initial value, By using the frame counter as a seed to the scrambler, the entire scrambler sequence is utilized. At system startup, the OLT initializes the frame counter to a specific start value.
FIG. 1 is an illustration of an existing XG-PON frame 11. The frame consists of a 24-byte frame header 12 and a frame payload 13. The frame header contains a physical layer synchronization field (PSYNC) 14 and may also contain a PON-ID field 15 and a frame counter field 16. The PON-ID field identifies the PON system, and in some systems is part of the unscrambled portion of the frame. The frame counter field, which increments by one for every frame, provides timing information to devices that are connected to these systems. It is also used as a seed (or sometimes referred to as a pre-load value) to the descrambler in the receiver. For every received frame, the receiver uses the seed to initialize the descrambler. The frame header may also contain parity bits (not shown) that provide error indications or error correction capabilities. There may also be other data fields within the frame header. The exact contents and position of the fields within the frame header is not yet standardized.
FIG. 2 is a simplified block diagram of an example of an existing frame-synchronous scrambler 17. The scrambler includes a 58-bit shift register 18 operating at the line rate. The scrambler scrambles all contents of the frame, apart from the initial header fields.